1. Field of the Invention
This invention relates generally to a method for controlling a regenerative braking event in a hybrid electric vehicle by changing the torque capacity of a torque converter lock-up clutch.
2. Description of the Prior Art
The fuel economy benefit in a hybrid electric vehicle results mainly from its ability to perform regenerative braking. In a hybrid electric powertrain an electric machine is coupled to the wheels through a torque converter and transmission gearing. The torque converter transmits torque through the combination of the hydraulic path and the mechanic path, provided the torque converter clutch is slipping. If the torque converter clutch is fully open, torque can only be transmitted through the hydraulic path. If the clutch is fully locked, the torque can only be transmitted through the mechanical path.
During regenerative braking, torque is transmitted from the vehicle wheels to the electric machine. If clutch is open, the torque converter's ability to transmit torque in the reverse direction is very limited. Any excessive regenerative torque can reduce the electric machine's speed. As a result, to recoup most of the kinetic energy using regenerative braking, the torque converter clutch should be kept locked while the vehicle is slowing down.
The torque converter clutch, however, must be opened for various reasons. When impeller speed is low, that clutch must be open so that the engine does not stall. When the clutch is open, the hydraulic path serves as coupling to deliver torque smoothly to the wheels.